In a double back stitch seam, the upper and lower threads are twisted at each stitch. With proper thread tension, the twisted area extends up to about the middle of the sewing material thickness, so that about the same pattern of a back stitch seam is formed on both sides of the sewing material. With a properly formed seam, each thread is so firmly anchored from stitch to stitch that the seam cannot be opened neither by tightening one end of the thread, nor by severing an individual stitch.
All known methods used for forming a double back stitch seam employ the same basic principle, where a limited lower thread supply is made available in the stitch forming area under the sewing material, and where the supply is passed completely through the upper thread loop and is replaced by a new limited supply after it has been used up. All known methods can be associated with one of two method groups. In one group the thread supply is moved with its carrier and its support through the upper thread loop at each stitch, with possible variations resulting from the paths and directions of motion during or between the stitch formations. The other, newer group of methods works with a stationary lower thread supply about which the upper thread loop, widened if necessary, is pulled by a looper element. All these known methods have the disadvantage, however, that the sewing must be interrupted when the lower thread supply must be replaced. This requires the formation of the corresponding thread supplies on suitable carrier bobbins etc. and suitable mechanisms; besides, there is a loss of time caused by the replacement, and a noticeable deviation from the seam pattern at least on the underside of the sewing material. Numerous methods and devices are known for trying to keep the above-mentioned disadvantages to a minimum. However, these disadvantages cannot be completely avoided in any of the known methods, and besides, the means used are elaborate and technically complicated and, hence, susceptible to trouble.
A sewing method is also known where, instead of a double back stitch seam, a so-called chain stitch seam or double chain stitch is formed. The simple chain stitch seam requires only an upper thread, the double chain stitch seam an upper and a lower thread. The stitches are formed by pulling each needle thread loop through the thread loop of the preceding stitch, and holding the following loop. In the simple chain stitch seam all loops are formed by the upper thread; in the double chain stitch seam the lower thread is laid by the upper thread guided in the above-described manner by means of a looper into a loop into which the needle penetrates during the next stroke and forms the upper thread loop there. Since in this method the lower thread need not be pulled completely through the upper thread loop, it is not necessary to wind a limited supply and to have it ready in the vicinity of the stitch. Chain stitch seams have the great disadvantage, however, that even if a single loop is open, that is, not anchored, the entire seam opens under a light pull. A single wrong stitch during the sewing thus yields an unusable seam. The same holds true for the slightest damage to the seam in a finished sewn product.